1. Field of the Invention
The present invention relates to signal processing apparatuses, signal processing methods, and programs, and more particularly to a signal processing apparatus, a signal processing method, and a program for obtaining characteristics of the distorted waveform of a signal.
2. Description of the Related Art
For example, in the case where wireless communication is performed using electromagnetic waves (radio waves) between a plurality of boards included in a predetermined apparatus in a housing, an electromagnetic wave transmitted from a transmitter board is reflected from walls of the housing or diffracted by other boards, and a receiver board receives the same electromagnetic wave transmitted via a plurality of paths.
The phenomenon that the receiver board receives the same electromagnetic wave transmitted via a plurality of paths is referred to as “multipath”. Multipath occurs not only in the housing, but also outside the housing due to obstacles, such as buildings and configuration of the ground.
In multipath, an electromagnetic wave is transmitted via a plurality of transmission paths having different distances, that is, a plurality of transmission paths involving different transmission times. Therefore, a signal value of a predetermined symbol of a signal received by the receiver board is affected by a plurality of symbols that are transmitted prior to the predetermined symbol and that are transmitted with delay via transmission paths with longer transmission times (i.e., electromagnetic wave interference).
In the case where a signal value of a predetermined symbol is affected by a plurality of symbols transmitted with delay, as has been described above, the waveform represented by the signal value of the predetermined symbol is distorted (multipath fading), and the receiver board has difficulty in accurately determining the values of the symbols of the signal. For example, the signal may not be accurately demodulated.
FIGS. 1A to 1H illustrate reception waveforms of electromagnetic waves in communication performed inside and outside a housing of a certain apparatus. The modulation scheme employed here is amplitude shift keying (ASK).
Four waveforms illustrated in FIGS. 1B, 1D, 1F, and 1H are reception waveforms obtained in communication performed inside the housing, and four waveforms illustrated in FIGS. 1A, 1C, 1E, and 1G are reception waveforms obtained in communication performed outside the housing. FIGS. 1A and 1B illustrate the reception waveforms at a transmission rate of 250 kbps. FIGS. 1C and 1D illustrate the reception waveforms at a transmission rate of 500 kbps. FIGS. 1E and 1F illustrate the reception waveforms at a transmission rate of 1 Mbps. FIGS. 1G and 1H illustrate the reception waveforms at a transmission rate of 2 Mbps.
For example, the most characteristic case illustrated in FIGS. 1G and 1H where the transmission rate is 2 Mbps will be discussed. The reception waveform outside the casing, which is illustrated in FIG. 1G, has “0” periods and “1” periods displayed well in order. In contrast, the reception waveform inside the housing, which is illustrated in FIG. 1H, is distorted since reflected waves overlap periods that should be “0”, so that these periods may be highly likely to be determined as “1”. In other words, the effects upon reception greatly differ depending on the transmission signal rate, and the quality of communication deteriorates significantly due to reflection from walls of the housing and the like.
Because of the effects of multipath fading, it is difficult to increase the communication path capacity and to maintain an arbitrary signal quality by performing simple signal processing.
In order to overcome the problems due to the foregoing effects of multipath fading, for example, the entire surface of the interior of the housing may be covered with an electromagnetic wave absorber, thereby suppressing reflection of electromagnetic waves from walls of the housing inside the housing (for example, see Japanese Unexamined Patent Application Publication No. 2004-220264). However, electromagnetic wave absorbers are expensive, and, in view of heat exhaustion, it is difficult to cover the entire surface of the interior of the housing with an electromagnetic wave absorber.
In order to handle the multipath effects in signal processing in general wireless communication of the related art, methods such as one using orthogonal frequency division multiplexing (OFDM), one using a spread-spectrum technique and rake reception, one using a multiantenna, and one using a waveform equalizer are available.
However, in the case where OFDM is employed as the modulation scheme, heavy load is placed on devices performing processing such as fast Fourier transform (FFT) and analog-to-digital and digital-to-analog conversion in modulation and demodulation, and these devices may exhaust more heat. In the case where a spread-spectrum technique is used, it is necessary to perform signal processing at a rate higher than the transmission signal rate, and high-speed communication becomes difficult to perform.
In the case where a multiantenna or a waveform equalizer is used, in view of the fact that the transmission characteristics may change according to time, changes in the transmission characteristics are difficult to predict, and noncorrelated noise may be superimposed on information to be transmitted, it becomes necessary to insert a unique word (UW) into a packet and to use a large-scale prediction circuit for improving the accuracy of predicting changes in the transmission characteristics. In the case where a multiantenna is used in a limited space inside the housing, cross-correlation occurs between the antennas, and it is regarded that no significant effects can be achieved to handle the multipath effects.
Even in the case where no effective measures can be taken against the multipath effects, as has been described above, if, for example, the characteristics of distortion of the waveform of a signal (statistical characteristics of multipath fading) in communication performed inside the housing can be obtained, the values of symbols of the signal can be accurately determined on the basis of the distortion characteristics, and it is expected that the quality of communication can be improved.